It has been postulated for many years that mammalian cardiac atria serve as sensors that are important in detecting changes in extracellular fluid volume (Gauer et al., Physiol, Rev. 43:423, 1963). Such a receptor function for the cardiac atria is known in the case of vasopressin, the hypothalmic hormone important in regulating the osmotic concentration of the body fluids.
The postulated existence of a substance which would enhance urinary sodium excretion, and hence be involved in regulation of extracellular fluid volume, was demonstrated recently. de Bold et al., Life Sci. 28:89, 1981, injected a partially purified extract of cardiac atria of rats into other anesthetized rats and observed a large increase in urine flow and in urinary sodium excretion. This relatively crude extract possessed the appropriate characteristics of an endogenous natriuretic substance.
In addition to its potent diuretic and natriuretic effects, properties that make the material especially appropriate to exert a major effect on body fluid volume regulation, it was also discovered that these extracts of cardiac atria have potent smooth muscle relaxant activity (Currie et al., Science 221:71, 1983). Such action implies a potential direct role in regulating blood pressure as well as a role in regulating extracellular fluid volume.
Because of the immediately recognized importance of this discovery for understanding the regulation of body fluid volume and blood pressure and the obvious therapeutic potential of such a natural substance in the treatment of congestive heart failure and hypertension, numerous laboratories set about to isolate, characterize and chemically identify the active substance(s) in the cardiac atrial extracts. The active substance(s) in cardiac atria was called atrial natriuretic factor or ANF but has been referred to also as cardionatrin (de Bold et al., Life Sci. 33:297-302, 1983), atriopeptin (Currie et al., Science 223:67, 1984) and auriculin (Atlas et al., Nature 309:717-719, 1984). Atrial natriuretic factor was shown to be a family of peptides all of which have a common amino acid sequence but differ in length by the presence or absence of 1-8 amino acids on the amino or carboxyl termini.